The need for increased yield and throughput in manufacture in general, and in the manufacture of semiconductor devices in particular, has driven the development and use of more highly automated article processing machines. An example of this trend is the increased use of cassette-to-cassette wafer processing machines in the manufacture of semiconductor devices. Such machines accept a cassette containing a plurality of semiconductor wafers, remove the wafers from the cassette and process them in a more or less automated fashion and return the wafers to the original cassette or to a second cassette.
An important part of any cassette-to-cassette wafer processing machine is the cassette elevator. The cassette elevator generally comprises a platform which carries the cassette and some means for moving the platform in a vertical direction. Since semiconductor wafers are carried in the cassette in a vertical stack, the precisely controlled vertical motion of the cassette elevator provides access to each of the wafers individually. In other words, the elevator is serially indexed so that each wafer in the cassette is brought into a position in which it can be removed from or inserted into the cassette by some wafer transfer mechanism which operates in a primarily horizontal fashion.
An example of a wafer transfer mechanism which operates in conjunction with the cassette elevator is a spatula transfer mechanism. A spatula transfer mechanism operates in a substantially horizontal plane and inserts a spatula capable of carrying a wafer into the open front side of the cassette. A wafer is removed from the cassette by indexing the cassette elevator to a position in which the spatula can be inserted immediately below that wafer. The spatula is inserted and the cassette elevator is indexed downward a small distance to rest the wafer on the spatula rather than on the cassette supports. The spatula is then removed, carrying the wafer.
A primary shortcoming of present cassette elevators is their inability to be configured in single or multi-elevator systems without substantially increasing the "footprint" and complexity of the system. In other words, prior art cassette elevators may only be positioned side by side with an individual spatula for each elevator. It has been impossible to place multiple cassette elevators front to back and serve them with a single spatula.
A further shortcoming of current cassette elevators concerns the method of loading and unloading cassettes onto the elevator platform. Typically, this operation is performed with the cassette platform in a horizontal position, this makes it relatively easy for wafers to be jostled out of the open front side of the cassette during the loading and unloading operation.